Light-emitting diodes (LEDs) are light-emitting elements made of semiconductor materials whose properties enable conversion from electric energy into light energy. LEDs have been massively used in lighting- and display-related applications due to their small physical volumes, long lifetimes, low driving voltages, short response times, and ready adaptability to various products in our daily lives.
FIG. 1 schematically shows the circuit structure of a conventional simple LED driver circuit 100, which includes a power source 11 and at least one LED 13. The power source 11 can generate a pulsed direct-current (DC) voltage for driving the LED 13.
Referring to FIG. 1 and FIG. 2, the input voltage VIN generated by the power source 11 varies periodically over time. When the input voltage VIN is higher than the forward bias voltage VF of the LED 13, the LED 13 can be successfully driven to emit light by the power source 11, as in the state of S=1 in FIG. 2. When the input voltage VIN is lower than the forward bias voltage VF of the LED 13, however, the LED 13 cannot be driven to emit light by the power source 11, as in the state of S=0 in FIG. 2.
Referring to FIG. 1 and FIG. 3, in order for the LED 13 to emit light continuously, one conventional approach is to add a capacitor 15 to the LED driver circuit 100. The capacitor 15 is charged by the power source 11 and thus stores a stored electric energy VC while the power source 11 is driving the LED 13. Therefore, the power supplied from the LED driver circuit 100 to the LED 13 will be a ripple voltage, which is the sum of the input voltage VIN and the stored electric energy VC (i.e., VIN+VC) and which can keep the LED 13 emitting light, as in the state of S=1 in FIG. 3.
However, despite the fact that the LED driver circuit 100 with the additional capacitor 15 can drive the LED 13 to emit light continuously, the aforesaid circuit design entails a waste of energy, for the capacitor 15 will keep discharging even when the power source 11 is at a relatively high potential level (e.g., when the input voltage VIN is higher than the forward bias voltage VF of the LED 13).
Moreover, in order to provide a sufficient driving power to the LED 13 incessantly, the ripple voltage (VIN+VC) must maintain at a relatively high level for a long time, and this cannot be done without a large-capacity capacitor 15 for energy storage. Nevertheless, as a large-capacity capacitor 15 generally can only be implemented by an electrolytic capacitor, which is both costly and prone to damage, the resultant LED driver circuit 100 will have a high circuit cost and less-than-satisfactory quality.